In the current changeover of refrigerants from chlorine-containing potentially ozone-depleting refrigerants to substantially non-chlorine-containing substantially non-ozone-depleting refrigerants, the refrigeration industry has been unable to replace refrigerant without necessitating major system changes. The simplest system change is substitution of a new expansion device. The most complex system change is a replacement of the system lubricating oil. Due to their lack of chlorine content, non-ozone-depleting replacement refrigerants are immiscible (i.e. insoluble) with lubricating oils presently circulated for compressor lubrication. Mineral oil is conventional for this purpose. It is miscible in refrigerants previously used but not in the new non-ozone-depleting refrigerants. Thus this advent of the replacement refrigerants has led to compressor failures due to inadequate return of immiscible oil.
There are two principal mechanisms for oil return in a refrigeration system: Refrigerant velocity and refrigerant solubility. By changing to oil-immiscible refrigerants, the solubility mechanism of oil return is removed. The refrigerant velocity mechanism is substantially unchanged when one utilizes conventional throttling expansion devices. The velocity mechanism of known oil return systems is simply insufficient to return oil with the replacement refrigerants, resulting in compressor failures.
The present conventional approach to the problem of oil return is to replace the oil when changing refrigerants, converting to oils which are miscible with the replacement refrigerants. Changing refrigerant system oil is an involved procedure calling for as many as four consecutive processes of flushing the system with the replacement oil, followed by a test of oil composition to insure relatively complete oil changeover. The process is protracted, in part due to the fact that the refrigerant oil is spread throughout the entire system and, since the old oil is non-soluble in the replacement refrigerant, there is no direct mechanism of washing out oil films from the system tubing walls.
Further problems with replacement oils, other than complete changeover, occur due to other chemical properties of the oils. The replacement oils are highly hydro-phillic, which simply means that they absorb water very easily and very quickly. Water vapor is a bane to refrigeration systems, and must be removed by pulling deep system vacuums prior to charging with refrigerant. The replacement oils, by absorbing water when exposed to ambient conditions, lead to unnecessary problems of water vapor presence within the refrigerant system. Water contamination of the replacement oils is difficult to detect visually, resulting in the recommended procedure of not using any replacement oil that has been exposed to ambient air. This leads to the discarding of any unused replacement oil which in general is extremely expensive.
A typical system changeover involves substituting R-12 with R-134a and replacing the lubricating mineral oil with polyol ester oil. The present invention enables a system changeover from R-12 to R-134a without requiring a change in lubricating mineral oil. R-12 and R-134a are recognized designations for refrigerants as deemed acceptable by EPA and provided in ASHRAE Standard 34.
Mineral oil miscible in the refrigerant in accordance with conventional practice deposits itself in a continuous thin film on interior parts of the system to achieve a good lubricating effect on the moving parts of the compressor. Conversion over to non-ozone-depleting refrigerants in which mineral oil is immiscible, while still using standard throttling devices to effect expansion, not only fails to move the oil satisfactorily throughout the system but also fails to achieve an effective continuous lubricating film on the compressor parts. Also, in failing to move the oil satisfactorily, pooling results in low parts of heat exchanger components of the system which effectively nullifies any heat exchange function where pooling occurs. It has been found, however, that by the practice of the method of this invention the high velocity bursts of refrigerant not only carry the immiscible oil throughout all stages of the process but deposit it in a film on the moving parts of the compressor comparable in good lubricating effect to the thin film achieved by mineral oil miscible in the refrigerant.